Adult T-cell leukemia/lymphoma (ATL) is a high grade malignancy caused by infection with the human T-cell leukemia virus type 1 (HTLV-I). ATL often rapidly progresses to death despite initial responses to chemotherapy. The Principal Investigator proposes to prepare novel gene therapy reagents for the treatment of ATL. Retroviral vectors exhibiting cytotoxicity for HTLV-I infected cells will be constructed and tested in preclinical studies for their ability to kill these cells. This approach is based on preliminary data from his laboratory demonstrating that it is possible to construct chimeric human immunodeficiency virus (HIV) which exhibit selective cytotoxicity for HTLV-I infected cells. Specific cytotoxicity, in this setting, was achieved by replacing the promoter/enhancer sequences in the HIV LTR with the HTLV-I Tax responsive elements (TREs), which mediate the transcriptional activation of HTLV Tax transactivator protein. This created a recombinant HIV that was transcribed and replicated selectively in the presence of HTLV Tax protein. The proposed studies will extend this observation to the creation of safer vector systems that will utilize the selectivity of retroviral replication imparted by Tax-TRE interactions. Two major approaches will be undertaken to develop replication-defective recombinant retroviral vectors that exhibit specific cytotoxicity for HTLV-I infected cells. First, the Principal Investigator will extend his initial studies and develop replication-defective, HIV-based vectors in which the expression of the herpes virus thymidine kinase (tk) gene is under the control of the chimeric HIV LTR containing the TREs. In the presence of ganciclovir, specific expression of TK, in response to the HTLV-Tax gene, should result in selective cytotoxicity of Tax expressing cells. He will also pursue strategies to design similar Tax-dependent cytotoxic vectors in murine leukemia virus (MLV)-based vectors. The presence of TREs in the MLV LTRs will direct specific TK expression to HTLV-I infected cells. In initial experiments with both these vector systems, he will demonstrate the specificity of cytotoxicity in HTLV-I infected cells and ATL cells. He will show that these recombinant viruses are not toxic to uninfected hematopoietic stem cells. An additional approach planned is to design specific retroviral vectors for ATL cells that will take advantage of the fact that these cells express high levels of the interleukin-2 (IL-2) receptor-alpha chain. Specific targeting of MLV vectors to these cells will be attempted by constructing retroviral envelope fusion genes and vector pseudotypes that will express IL-2 on the virion surface. He proposes that these vectors will have an eventual use in gene therapy for ATL. The final aspect of the proposal is to perform preclinical studies to assess the potential efficacy of these recombinant retroviruses in purging ATL cells from harvested hematopoietic stem cells, and thus to allow for autologous transplantation following myeloablative therapy for ATL.